[Ultrasonography of placenta previa at the third trimester of pregnancy: research for signs of placenta accreta/percreta and vasa previa. Prospective color and pulsed Doppler ultrasonography study of 45 cases].

OBJECTIVES: In a population of 45 placenta previa observed at third trimester of gestation we have tried to make the prenatal diagnosis of placenta percreta and vasa previa with color and pulsed Doppler ultrasonography.

MATERIALS AND METHODS: We used a transabdominal sonography with full bladder and a transvaginal sonography with empty bladder. The first images obtained showed 20 placenta previa over the internal cervical os, 3 marginal and 22 low-lying placenta previa. We tried to find evidence of placenta percreta with gray-scale ultrasonography (loss of normal hypoechoic retroplacental myometrial zone, focal disruption of the uterine serosa and surrounding tissues, presence of intra placental lacunae) and with color and pulsed Doppler (arterial vessels with a diastolic flow value less than the flow value of a spiral artery behind the placenta, arterial vessels crossing from the placenta to surrounding tissues, intraplacental lacunae with arterial flow). We tried to find evidence of vasa previa in color and pulsed Doppler (a fetal vessel in seen above the lower segment of the uterus and below the fetal head. There is no change in the location of the vessel despite positional changes in both mother and fetus). The positivity of one sign in gray-scale ultrasonography or in color and pulsed Doppler led us to believe that the patient was affected by the anomaly. The final diagnosis of abnormal adherence of the placenta and of vasa previa was made on histological examination.

RESULTS: Among the 20 placenta previa over the internal cervical os, we found 1 placenta percreta, 1 placenta accreta and 3 cases of vasa previa. In spite of our limited sample of cases of abnormal adherences, our results showed that gray-scale ultrasonography was sufficient to make a prenatal diagnosis of placenta accreta/percreta. Negative predictive value is 100% on a sample of 43 patients with no abnormal placental adherence. Color and pulsed Doppler brought no further evidence. In our population, color and pulsed Doppler had 100% positive predictive value for diagnosis of vasa previa.

CONCLUSION: We found the only 2 cases of abnormal adherent placental fragments and the 3 cases of vasa previa present in our population. The study of the lower segment of the uterus should be thorough as abnormal zones may be small-sized. Color Doppler is the reference technique for sighting vasa previa and gray-scale ultrasonography for abnormal adherences of the placenta.